Defines the earliest phase within the period at which the rule should
execute; the scheduler attempt to find an optimal phase from 0 <= n <
period (thus, the phase must be at least zero and less than the current
period.).

Assertions and Functional Coverage

An assertions checks that an E Bool is true. Assertions are checked
between the execution of every rule. Parent enabling conditions can
disable assertions, but period and phase constraints do not. Assertion
names should be globally unique.